VGAX49

Features

Digital VGA adapter
16-bit digital video
- 5:6:5 (R:G:B) mapping (maximum)
- 2:2:2 (R:G:B) mapping (option)
Uses summing resistors
DB-15F connector
49x49mm ODAS form factor
Mounting holes

Assumptions
- 3.3V Drive out of FPGA
- 1V is drive level for VGA specification
- 8 mA max drive current (typical FPGA drive current)
VGA presents a 75 Ohm load
Series resistors present a voltage divider between the resistors on the card and the VGA load
1V at 75 Ohms is 13.3 mA
Each color is independently driven
The resistor with the smallest resistance value has larger current from the FPGA pin
Current switches from source to sink depending on the voltages on the other resistors
Use common value, 1% resistors
330 Ohm series resistor into a 75 Ohm load is 8.15 mA
- Voltage with 330/75 ohm voltage divider is 0.61V - not full brightness
Each resistor is 2X the value of the previous resistor
Next standard value from 330 ohms is 680 ohms
- Voltage is 0.83V - better brightness

Assumptions
- 3.3V Drive out of FPGA
- 1V is drive level for VGA specification
- 8 mA max drive current (typical FPGA drive current)
VGA presents a 75 Ohm load
Series resistors present a voltage divider between the resistors on the card and the VGA load
1V at 75 Ohms is 13.3 mA
Each color is independently driven
The resistor with the smallest resistance value has larger current from the FPGA pin
Current switches from source to sink depending on the voltages on the other resistors
Use common value, 1% resistors
330 Ohm series resistor into a 75 Ohm load is 8.15 mA
- Voltage with 330/75 ohm voltage divider is 0.61V - not full brightness
Each resistor is 2X the value of the previous resistor
Next standard value from 330 ohms is 680 ohms
- Voltage is 0.83V - better brightness
Voltage with all 6 resistors (Green case) is 1.07V (full brightness)